Linq to update a collection with values from another collection?

Question

I have IQueryable<someClass> baseList

and List<someOtherClass> someData

What I want to do is update attributes in some items in baseList.

For every item in someData, I want to find the corresponding item in baselist and update a property of the item.

someOtherClass.someCode == baseList.myCode

can I do some type of join with Linq and set baseList.someData += someOtherClass.DataIWantToConcantenate.

I could probably do this by iteration, but is there a fancy Linq way I can do this in just a couple lines of code?

Thanks for any tips, ~ck in San Diego

Answer 1

LINQ is for querying - not for updating. That means it'll be fine to use LINQ to find the corresponding item, but for the modification you should be using iteration.

Admittedly you might want to perform some appropriate query to get baseList into an efficient form first - e.g. a Dictionary<string, SomeClass> based on the property you'll be using to find the corresponding item.

Answer 2

You can convert the IQueryable<SomeClass> into a List<SomeClass>, use the ForEach method to loop over it and update the elements, then convert back to IQueryable:

List<SomeClass> convertedList = baseList.ToList();
convertedList.ForEach(sc =>
{
    SomeOtherClass oc = someData.First(obj => obj.SomeCode == sc.MyCode);
    if (oc != null)
    {
        sc.SomeData += oc.DataIWantToConcatenate;
    }
});

baseList = convertedList.AsQueryable(); // back to IQueryable

But it may be more efficient during this using non-LINQ constructs.

Answer 3

You can't simply find objects that are in one list but not the other, because they are two different types. I'll assume you're comparing a property called OtherProperty that is common to the two different classes, and shares the same type. In that case, using nothing but Linq queries:

// update those items that match by creating a new item with an
// updated property
var updated =
 from d in data
 join b in baseList on d.OtherProperty equals b.OtherProperty
 select new MyType()
 {
  PropertyToUpdate = d.PropertyToUpdate,
  OtherProperty = d.OtherProperty
 };

// and now add to that all the items in baseList that weren't found in data
var result =
 (from b in baseList
  where !updated.Select(x => x.OtherProperty).Contains(b.OtherProperty)
  select b).Concat(updated);

Answer 4

To pair elements in the two lists you can use a LINQ join:

var pairs = from d in someData
            join b in baseList.AsEnumerable()
                on d.someCode equals b.myCode
            select new { b, d };

This will give you an enumeration of each item in someData paired with its counterpart in baseList. From there, you can concatenate in a loop:

foreach(var pair in pairs)
    pair.b.SomeData += pair.d.DataIWantToConcantenate;

If you really meant set concatenation rather than +=, take a look at LINQ's Union, Intersect or Except methods.